Mycoplasma pneumonia is atypical pneumonia caused by Mycoplasma pneumoniae. Mycoplasma pneumonia, together with chlamydia pneumonia, constitutes 30% to 40% of the atypical pneumonia cases and also constitutes a high percentage of the community-acquired pneumonia cases.
Mycoplasma pneumonia is common in infants, children, and adolescence. The incubation period is 2 to 3 weeks. Excretion of the pathogen into respiratory mucosa is observed in 2 to 8 days before the onset of initial symptom, becomes the highest at the onset of clinical symptoms, continues at a high level for about one week, and then further continues for 4 to 6 weeks or more. Main clinical symptoms are fever, general malaise, headache, and other cold-like symptoms. Mycoplasma pneumonia is characterized by, for example, high fever greater than 38° C. and intense dry cough. The cough further continues for a long time, 3 to 4 weeks, after decline of fever. However, there is no examination finding characteristic to mycoplasma pneumonia, and pale ground-glass appearance in chest X-ray examination is typical.
The manner of infection with Mycoplasma is droplet infection and contact infection from an infected patient. Mycoplasma pneumoniae invades the respiratory tract and attaches to the bronchi or bronchiole epithelium to achieve the infection.
Mycoplasma infection is designated as a notifiable infectious disease (Infectious diseases Category V) based on the Infectious Diseases Control Law, and designated medical care providers have an obligation to promptly report the number of patients.
Mycoplasma is a minimum microorganism that can replicate itself, and differs from other bacteria in that it does not have a cell wall. Accordingly, β-lactam antibiotics and cephem antibiotics, which are antibiotics having a function of inhibiting cell wall synthesis, are ineffective, and an administration of macrolide antibiotics, tetracycline antibiotics and new quinolone antibiotics is required for treatment. Prompt identification of the pathogen is therefore necessary for determining the initial treatment plan.
At present, Mycoplasma pneumoniae infection is definitely diagnosed by an isolation culture method and a serological test.
The isolation culture needs a specialized culture medium (PPLO medium) for detecting Mycoplasma. In addition, its proliferation is slow, compared to other bacteria, thereby taking at least about one week for obtaining the result of determination. It is therefore difficult to rapidly identify the pathogen by the isolation culture method in clinical sites.
Mycoplasma is susceptible to temperature, and samples containing Mycoplasma cannot be kept in cold storage, unlike samples containing common bacteria. Accordingly, Mycoplasma contained in a sample may become extinct or decrease during storage or transportation of the sample and may not be detected even by the isolation culture method.
Examples of the serological test include an indirect hemagglutination (IHA) test, a particle agglutination (PA) method, and an enzyme immunoassay (EIA), which specifically detect IgG antibody or IgM antibody.
Furthermore, an immunochromatographic kit (ImmunoCard Mycoplasma Antibody, available from TFB, Inc.) is commercially available as a simple test detecting Mycoplasma pneumoniae-specific IgM antibody in serum or plasma by EIA and is used at clinical sites.
In the serological test, although the IgM antibody in a sample to be detected increases at the early stage of infection, the sample may show false negative in the case of low antibody production response or depending on assay timing. Furthermore, since it takes a long time before IgM antibody disappears in blood, it cannot be said that the result of the serological test always correctly indicates the current infection status.
Accordingly, definite diagnosis by the serological test needs quantitative tests using paired sera of acute and convalescent stages, and therefore has to be ex-post diagnosis in many cases.
A nucleic acid detection method for detecting DNA of Mycoplasma pneumoniae is also employed. In the detection method, however, the procedure of amplifying nucleic acid is complicated and needs specialized equipment, and the assay takes several hours. Thus, the method is not a test that is generally used.
In order to more rapidly and simply detect Mycoplasma pneumoniae infection, specific antibody against Mycoplasma pneumoniae antigen has been developed, and a detection method of distinguishing whether mycoplasma infection is present or not has been reported.
Mycoplasma pneumoniae attaches itself to the respiratory epithelial cell cilia with its adhesive organ in a form of a flask-shaped protrusion, and then moves to the cell surface by gliding motility and adheres thereto to achieve the infection. Production of an antibody specific to P1 protein (169 KDa), which is known as adhesive protein playing a central role in this adhesion or gliding motility, and a detection method using the P1 protein as a detection marker have been reported (Patent Documents 1 and 2).
It is also known that P1 protein, the antigen to be detected reported in the above reports, has two genotypes and that the amino acid sequences corresponding to the genotypes of P1 protein are different from each other. Accordingly, in order to broadly detect Mycoplasma pneumoniae, production of each antibody against P1 protein of each P1 genotype or an antibody recognizing a common site of P1 protein of the different genotypes is necessary. In addition, seasonal epidemic has been reported such that a genotype different from the epidemic genotype in the latest season is detected, that is, the genotype changes depending upon epidemic seasons. Therefore, it is necessary to find out the genotype at an early stage of the epidemic and use an antibody specific thereto.
A detection method using DnaK protein, which is known to be conserved among isolated strains of Mycoplasma pneumoniae compared with P1 protein, as a detection marker (Patent Literature 3) has been also reported. However, Mycoplasma genitalium causing human urinary infectious diseases also has DnaK protein, and therefore also shows reactivity in the detection method. Consequently, Mycoplasma pneumoniae cannot be specifically detected by using the above protein as a detection marker.
If mycoplasma pneumonia is not appropriately treated, the symptoms may be protracted or become severe or may further cause the spread of the infection due to secondary infection. Accordingly, in order to select appropriate treatment and antibiotics, rapid and conclusive detection of Mycoplasma pneumoniae is demanded.
Furthermore, even though specific and rapid detection of Mycoplasma pneumoniae has been contemplated, a detection marker that can further specifically detect Mycoplasma pneumoniae, a specific antibody against the marker, and also an immunoassay and a kit containing the antibody have been demanded.